3.14.1.1 and 3.12.1.0 build failure: Installation directories are not prefix_relative: internal error InstallDirs.libsubdir

**Describe the bug**
Build failure with numerous of errors like this:
```
Failed to build base16-bytestring-1.0.2.0. The failure occurred during the
configure step.
Build log (
/opt/local/var/macports/build/_Users_runner_work_macports-ports_macports-ports_ports_lang_cabal/cabal/work/.home/.cabal/logs/ghc-9.6.6/bs16-bytstrng-1.0.2.0-7247b4c4.log
):
Configuring library for base16-bytestring-1.0.2.0...
Error: [Cabal-6000]
Installation directories are not prefix_relative:
internal error InstallDirs.libsubdir
CallStack (from HasCallStack):
error, called at src/Distribution/Simple/InstallDirs.hs:146:19 in Cabal-3.14.1.1-inplace:Distribution.Simple.InstallDirs
```
See CI logs [here](https://github.com/macports/macports-ports/pull/27341) for complete build logs

**To Reproduce**
Steps to reproduce the behavior:
Basically trying to build cabal 3.14.1.1 or 3.12.1.0 from source, using official cabal binary

**Expected behavior**
It builds

**System information**
- macOS 15
- cabal 3.14.1.1 or 3.12.1.0, using ghc 9.12.1 and 9.6.6

**Additional context**
Could be somehow related to 7b90583 but I'm not sure.
Build args:
`--project-file=cabal.release.project --bindir=/opt/local/bin --datadir=/opt/local/share/cabal`
`.cabal/config` file:
```
relocatable: True
install-method: copy
installdir: /opt/local/bin

install-dirs global
prefix: /opt/local
bindir: /opt/local/bin
libdir: /opt/local/lib
libsubdir: cabal
dynlibdir: /opt/local/lib
libexecdir: /opt/local/libexec
libexecsubdir: cabal
datadir: /opt/local/share/cabal
docdir: /opt/local/share/doc/cabal
htmldir: /opt/local/share/doc/cabal
haddockdir: $htmldir
sysconfdir: /opt/local/etc/cabal

program-locations
gcc-location: /usr/bin/clang
```